Pickling is the chemical removal of surface oxides or scale from metals by immersion in an aqueous acid solution. For example, solutions containing mixtures of nitric acid and hydrofluoric acid are employed for pickling stainless steels, titanium, zirconium and other metals that are corrosion resistant. These pickling solutions become contaminated with dissolved metals through use. As the metal concentration increases, the free acid concentration decreases and pickling efficiency drops. Additions of fresh concentrated acid are made from time to time to rejuvenate the bath, but eventually it becomes spent and must be discarded.
Although many mineral acids such as sulfuric, hydrochloric and nitric acid are relatively inexpensive, hydrofluoric acid is considerably more expensive, so that disposal of pickle liquors containing fluoride represents a significant loss in terms of the value of the contained fluoride.
Disposal of spent pickling solutions is becoming increasingly difficult and expensive. It is no longer considered environmentally acceptable to discharge spent pickling solution directly into municipal sewers or watercourses and the availability of deep well disposal sites is becoming limited. Discharge of fluoride and nitrate ions is strictly controlled in many regions. Transport of spent pickling solution is also becoming difficult and costly, as spent pickling solution is classified as a hazardous substance whose transport is strictly controlled.
Many pickling operations neutralize spent pickle liquors with an alkali such as sodium hydroxide (caustic soda) or calcium hydroxide (lime). In the case of fluoride containing pickle liquors, calcium hydroxide is usually utilized. Calcium fluoride is only slightly soluble, so that fluoride ions are removed simultaneously with the metal ions, which are precipitated. Unfortunately, neither lime nor sodium hydroxide are effective in removing nitrate ions. The cost of these neutralizing chemicals is considerable and can contribute appreciably to the overall cost of pickling the metal.
Recently, the disposal of the resulting sludges has become a particular concern. These sludges are considered hazardous waste and as such, their disposal has become severely restricted and very expensive. It is becoming widely recognized that a more sensible approach to the problem of disposal of hazardous solid waste is to reclaim the metal values. In the case of metal hydroxide sludges, pyro-metallurgical technology for converting them back to metals is well understood and is being practised today. This approach is particularly attractive for stainless steel pickling operations since sludges emanating from these operations typically contain appreciable quantities of chromium and nickel, which possess significant potential economic value. Unfortunately, the presence of fluoride in these sludges is considered deleterious to the sludge recovery process. As a result, it is not generally feasible to reclaim sludges emanating from pickling operations employing hydrofluoric acid.